Github user nsyca commented on the issue:
https://github.com/apache/spark/pull/14719
The test case @sarutak raised here is what I consider the problem of the
current code.
```df.join(df, df("key") === df("key"))```
How do we make a conclusion that the left operand df("key") references the
first relation df and the right operans the second relation df? We can't.
Arguably, we can treat this predicate as a local predicate in which both
operands reference one of the two relations, that is, it could mean any of the
three cases below:
1) df.join(df.as("df2"), df("key") === df2("key")
2) df.join(df.as("df2"), df("key") === df("key")
3) df.join(df.as("df2"), df2("key") === df2("key")
Hence this type of statement should yield an ambiguous reference error.
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